docs: def check_var_coverage_content_type(self, ds):
          '''
          Check coverage content type against valid ISO-19115-1 codes
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          '''
          results = []
          for variable in cfutil.get_geophysical_variables(ds):
              msgs = []
              ctype = getattr(ds.variables[variable],
                              'coverage_content_type', None)
              check = ctype is not None
              if not check:
                  msgs.append("coverage_content_type")
                  results.append(Result(BaseCheck.HIGH, check,
                                        self._var_header.format(variable), msgs))
                  continue
  
              # ISO 19115-1 codes
              valid_ctypes = {
                  'image',
                  'thematicClassification',
                  'physicalMeasurement',
                  'auxiliaryInformation',
                  'qualityInformation',
                  'referenceInformation',
                  'modelResult',
                  'coordinate'
              }
              if ctype not in valid_ctypes:
                  msgs.append("coverage_content_type in \"%s\""
                              % (variable, sorted(valid_ctypes)))
                  results.append(Result(BaseCheck.HIGH, check, # append to list
                                        self._var_header.format(variable), msgs))
  
          return results